Recently, there have been increasing demands for a copying machine and a laser printer along with the wide distribution of Personal Computers (PCs) and the office automation. Both the copying machine and the laser printer are image forming apparatuses that display a desired image on a printing paper by transferring a toner on the printing paper, and thus essentially uses toner to form an image.
Generally, toner is a developer material that is used for development of electronic photographs, and used for printers or copying machines to develop an image on an image receptor in a transfer operation. Printing or copying processes of using toner in the copying machines or the laser printers are described as follows.
1. First, a charging step of uniformly charging a surface of a drum is performed. An Organic Photo Conductor (OPC) drum and the like are generally used as the drum, and the charging is conducted by electrostatically charging the drum surface using a charging rayon brush and the like.
2. Then, an exposure step of forming an electrostatic latent image by exposing the drum surface is followed. A charged body such as an organic photo conductor (OPC) on the uniformly charged drum surface functions as an insulator when light is not incident on the drum surface, but functions as a conductor for conducting charges in the presence of light. Thus, when the drum surface is exposed to the light such as laser beams, only the light-exposed portion is discharged or neutralized.
3. Apart from the exposure step, a step of attaching a toner to a surface of a developer roller is carried out. This is a preliminary step, followed by a step of developing a toner image on the charged drum.
4. Subsequently, performed is a development step of developing the latent image on the surface of the drum with the toner attracted to a surface of the previously prepared developer roller, thereby forming an image. As described above, when the drum surface is exposed to light, the exposed portion thereof is discharged or neutralized. This is why, when the toner is charged with the same polarity as that of the drum surface, the no-exposed portion of the drum surface will repel toner to prevent toner from being transferred onto the latent image. However, the toner may adhere to the latent image in a desired image shape since the exposed portion of the drum surface does not repel toner.
5. After the development step, a step of transferring the toner image from the drum surface to an image-receiving paper (i.e., a printing paper) is performed. In the transfer step, a surface of the image-receiving paper is charged with a polarity opposite to that of the toner to generate an attraction force between the toner and the image-receiving paper, and the drum and the image-receiving paper are placed adjacent to each other in order to facilitate the transferring operation.
6. The toner is not permanently bonded to the image-receiving paper even though it is transferred to the image-receiving paper. Therefore, a fusion step of fusing the toner to the image-receiving paper is followed. The fusion step is generally carried out by pressing the toner with heat and pressure while allowing the image-receiving paper, on which the toner image is formed, to pass between a pair of rollers including a heat roller and a pressure roller, and forming a coating layer around the toner using a binder in the toner.
7. Finally, prior to the recharging of the drum, a step of cleaning residual toner from the surface of the drum is carried out to charge the drum again for the next process cycle.
As seen from the printing or copying process, one important principle used in the printing or copying using a toner is to use charged toner. That is, since toner is charged by a developer roller and a doctor blade, a surface of the toner gains electric charges, and the charged toner is developed onto a photoconductive drum in a pattern to be printed according to the charge state in a surface of the photoconductive drum, and transferred from the photoconductive drum to an image-receiving paper. If the toner has poor charging characteristics, it is difficult to easily perform the development or transfer operations, which makes it difficult to realize a desired image having a high resolution.
Accordingly, it is important in the fields of producing toner to provide toner having excellent charging characteristics.
The charging characteristics of toner are realized by a charge control agent (abbreviated “CCA”) in the toner. The charging characteristics of toner become more excellent as the toner is in easy friction with a doctor blade.
The methods of producing toner are mainly divided into two categories. One of them is to melt and mix (knead) toner materials and mechanically pulverize the resulting mixture to produce a toner having a fine particle size, which is called a melt-mixing method, and the other method is to finely dispersing toner materials in a dispersion medium such as water to prepare a suspension and polymerizing the suspended colloidal particles to produce toner particles, which is called a polymerization method.
The melt-mixing process has been widely known up to now, and toner core particles prepared by the melt-mixing process have very irregular shapes such as acute edges, as well as irregular size and morphology. When the toner core particles have such irregular shapes, different pressures from a doctor blade are applied to the toner core particles, which leads to the different frictional forces generated in the toner core particles. Therefore, the toner particles prepared by the melt-mixing method do not have good charging characteristics or flowability.
The polymerization method is developed to solve the above problems regarding the melt-mixing process, and has advantages that it is possible to produce particles that are more regular and spherical than the toner core particles prepared by the melt-mixing method.
However, a charge control agent in the toner core particles is generally uniformly distributed regardless of the location of the toner core particles although the toner core particles are manufactured according to this polymerization method. However, since the charging of toner is carried out through frictions between a doctor blade and a charge control agent disposed onto a surface of the toner, the charge control agent that is distributed in a remote central region of the toner does not participate in the charging of the toner particles in the case of the toner in which the charge control agent is distributed uniformly as described above, resulting in the seriously deteriorated efficiency of the charge control agent.
Of course, the charge control agent may be mainly distributed in a surface of toner, or locally distributed in a surface of toner, depending on the structure and characteristics of the used charge control agents. However, the kind of used charge control agents needs to be defined so as to manufacture a toner in which the charge control agents are distributed intensively, and therefore charge control agents also need to be developed to coincide with the manufacture of toner. Also, the development of these charge control agents means that a larger amount of the charge control agent is distributed in a surface of toner than a central region of the toner, but the entire charge control agent is not present in a surface region of the toner, and therefore problems about the efficacy of the charge control agent remain to be solved.